Loop antenna

A loop antenna is a (up to about 30 cm) mounted generally on a frame with large cross-section air-core coil , which as a receiving antenna for Very long , long , medium and short wavelength is used. It is a magnetic antenna , so primarily generates or receives the magnetic part of the electromagnetic field.

Loop antenna from the early days of radio reception

The coil of wire is inductance L, it must be connected to a capacitor C.

Loop antenna for detector reception

Layout and function

Since the circumference is usually much smaller than half a wavelength, a loop antenna has a homogeneous current distribution along the conductor, unlike a half-wave dipole, which has a voltage maximum at the dipole ends and a current maximum in the middle. Their base point resistance is complex in principle and, depending on the number of turns, is between 130 and less than 1 ohm. Loop antennas can either have a broadband design and are then relatively insensitive or they can be tuned by connecting a variable capacitor in parallel. Tuned antennas are more sensitive due to the increased resonance , but only for one frequency. Matched loop antennas have very high real base point resistances.

Like all magnetic antennas, the loop antenna has a directional effect - a coil with a horizontal winding axis has a figure-of-eight characteristic in the horizontal plane for vertically polarized waves and does not receive any horizontally polarized waves. Since waves from radio towers / masts are polarized vertically, the antenna only needs to be rotated around the vertical axis in order to be aligned with the transmitter - its winding axis is then perpendicular to the receiving direction.

In terms of its theory, the loop antenna is a very direct illustration of the induction principle , according to which the induced voltage is proportional to the change in the magnetic flux: Here the area A is particularly large (and usually also the number of turns n) in order to create a large effect (a high induction voltage ) to achieve. The change in magnetic flux comes from the electromagnetic radiation arriving from the transmitter; their electric field, magnetic field and the direction of propagation are perpendicular to each other.

For transmission with a loop antenna, a high-frequency current in the coil itself generates an alternating magnetic field in space and, as a result, an electromagnetic wave.

Loop antennas can be installed in an interrupted metal pipe ring and are then largely insensitive to electrical interference fields or lightning strikes.

Applications

Loop antennas were used, at least earlier, for shortwave radio on ships, whereby their directional effect was additionally exploited by a rotatable assembly. Rotatable loop antennas are used for radio direction finding. The received signal has a maximum precisely when the coil plane points in the direction of the transmitter. If the coil is perpendicular to the receiving direction, it is zero.

There are also a few radio receivers that do not have a ferrite antenna as a receiving antenna for the AM range, but a small loop antenna (diameter here only approx. 10 cm) that is set up separately next to the device and can be aimed at the desired transmitter.

Self-made large loop antennas are mainly used today among radio listeners in the medium wave range and among radio amateurs in the short wave range. These loop antennas are up to one and a half meters in size and are operated on world receivers or their own stations. Loop antennas can also be found in RFID transponders. Another application is measurements of electromagnetic compatibility .

Basic equations

The terminal voltage U _e for the loop antenna results approximately from the following equation:

${\displaystyle U_{\mathrm {e} }\approx {\frac {2\pi nA}{\lambda }}E}$

with E for the incident electric field strength [V / m], λ for the wavelength of the radiation [m], A for the antenna area [m²] and n for the number of turns .

For the radiation resistance R _s [Ohm] the following applies approximately:

${\displaystyle R_{\mathrm {s} }\approx {\frac {2}{3}}Z_{\mathrm {0} }\pi \left({\frac {2\pi nA}{\lambda ^{2}}}\right)^{2}}$

and for the inductance L [H] as an approximation:

${\displaystyle L\approx \mu _{0}n^{2}{\frac {d_{\mathrm {r} }}{2}}\ln \!\!\left({\frac {d_{\mathrm {r} }}{d_{\mathrm {D} }}}\right)}$

With as the diameter of the frame [m] and , the diameter of the wire [m] and the magnetic field constant µ _o. ${\displaystyle d_{\mathrm {r} }}$ ${\displaystyle d_{\mathrm {D} }}$

From this, under the assumption that the wire length is, the generator power P _gmax that can be drawn from the resistance _layer k 'of the wire can be _written . ${\displaystyle l<{\frac {\lambda }{6}}}$

${\displaystyle P_{\mathrm {gmax} }={\frac {\pi ^{3}nd_{\mathrm {r} }^{3}}{16k'\lambda ^{2}}}*E^{2}}$

The following derivations can be formulated from these equations:

• The line resistance far outweighs the radiation resistance in all practical designs.
• The generator power that can be drawn can be greatly influenced by the area of ​​the antenna. The number of turns n is only included linearly in the result.
• The degree of efficiency is extremely low, especially with structures with a small area.

The use of loop antennas is therefore only sensible in the range of low frequencies and high field strengths (> 20 μV / m). It is unsuitable as a transmitting antenna because of its low efficiency. At higher frequencies, stranded wire instead of wire improves the magnetic properties of the coil.

Special shape

Loop antenna with tuner for 1.75 to 30 MHz (diameter 2 m)

A special form of the loop antenna is the so-called loop . This is a loop antenna with usually only one, and occasionally two turns. Due to the lower inductance of the coil, its resonance frequency is significantly higher than that of the loop antenna and can reach into the GHz range.

literature

• Ulrich Freyer: Antenna technology for radio practitioners. 1st edition, Franzis-Verlag GmbH, Poing 2000, ISBN 3-7723-4693-6
• Gerd Klawitter: Antenna advisor receiving antennas for all wave ranges. 6th edition, Verlag für Technik und Handwerk, Baden-Baden 2005, ISBN 3-88180-613-X

Web links

Commons : Loop antennas  - collection of pictures, videos and audio files

• Calculation and assembly instructions for an AM loop antenna
• AM loop antenna and matching antenna amplifier or FET antenna amplifier
• switchable loop antenna for MW and LW

Individual evidence

1. ↑ Zinke, O., Brunswig: high frequency technology 1 . High frequency filters, lines, antennas. Springer-Verlag, 5th edition.